1. Field of the Invention
The present invention relates to a display apparatus for displaying large-screen images, and more particularly to an integrator which can improve optical efficiency by simplifying the structure of an optical system and minimizing optical loss, and a polarization conversion device using the same.
2. Background of the Related Art
Recently, a plate-type display apparatus which is thin and suitable for large-screen images has attracted attention instead of a CRT (Cathode Ray Tube) which is restricted in screen size and which has a large-sized system. Exemplary plate-type display apparatuses include LCD (Liquid Crystal Display, PDP (Plasma Display Panel) and projector.
Among the large-screen image display apparatuses, the projector displays color images by enlarging and projecting small images of a small-sized display device into a large screen through a projecting lens. It has been expected to replace the other large-screen image displays apparatuses.
In general, the projector includes a lamp system for generating lights, an optical system for condensing the lights, a display device for displaying the condensed lights after modulating the optical property thereof according to an electric signal, and a projection optical system for enlarging and projecting the modulated colors. Here, the display device is classified into a transmission type for transmitting the incident lights and a reflection type for reflecting the incident lights according to the electric signal. In addition, a transmission or reflection type LCD which is advantageous in miniaturization is generally used as the display device.
The projector is divided into a single-plate type using one display device, a two-plate type using two display devices, and a three-plate type using three display devices.
For example, the three-plate type display device is used for high luminance, and the single-plate type display device is used for miniaturization and light weight.
FIG. 1 is a view illustrating an optical system for a conventional projector.
Referring to FIG. 1, the optical system includes a lamp system 1 for generating lights, and a polarization conversion system 5 for polarizing and outputting the lights from the lamp system 1.
The lamp system 1 has a bulb 2 for generating lights, and a reflecting mirror 3 for outputting the lights from the bulb 2 in parallel.
The polarization conversion system 5 is composed of a first optical device 7, a second optical device 10 and a second illuminating lens 16.
In the first optical device 7, a plurality of luminous flux splitting lens 8 formed in a rectangular shape are aligned in a matrix form. Preferably, the first optical device 7 is a fly eye lens.
The second optical device 10 which is a polarization conversion device includes a fly eye lens 11, a polarization splitting unit array 13, a wave plate 14 and a first illuminating lens 15.
Here, the lights generated in the lamp system 1 are inputted to the second optical device 10 via the first optical device 7. The lights inputted to the second optical device 10 are spatially split into P polarized lights and S polarized lights by the polarization splitting unit array 13, and incorporated by the wave plate 14.
The incorporated polarized lights are irradiated to the second illuminating lens 16 via the first illuminating lens 15.
FIG. 2 is a view illustrating a projection system using a general three-plate transmission type LCD for the conventional projector.
As illustrated in FIG. 2, the projection system includes a second illuminating lens 16 for condensing the lights from the bulb to a panel, R, G and B reflecting mirrors 17, 18 and 19 for splitting the lights from the second illuminating lens 16 into color lights, reflecting mirrors 20 and 21 for re-reflecting the lights from the R, G and B reflecting mirrors 17, 18 and 19, R, G and B LCDs 22, 23 and 24 for modulating the lights from the reflecting mirrors 20 and 21 to add image information to each color light, a PBS (Polarizing Beam Splitter) 25 for forming color images by synthesizing the modulated lights from the R, G and B LCDs 22, 23 and 24, and a projecting lens 26 for enlarging and projecting the color images from the PBS 25 into a screen 27.
In the projection system, the lights from the second illuminating lens 16 are inputted to the R reflecting mirror 17. The B and G lights are transmitted through the R reflecting mirror 17, and the R lights are reflected by the R reflecting mirror 17. The R lights reflected by the R reflecting mirror 17 are re-reflected by the reflecting mirror 20, and inputted to the R LCD 22.
On the other hand, the B lights transmitted through the R reflecting mirror 17 are transmitted through the G reflecting mirror 18, and the G lights transmitted through the R reflecting mirror 17 are reflected by the G reflecting mirror 18.
The G lights reflected by the G reflecting mirror 18 are inputted directly to the G LCD 23.
In addition, the B lights transmitted through the G reflecting mirror 18 are reflected by the B reflecting mirror 19 and the reflecting mirror 21, and inputted to the B LCD 24.
Here, the R, G and B LCDs 22,23 and 24 modulate the inputted color lights to add image information to each color light, and transmit them to the PBS 25.
The PBS 25 receives the modulated lights from the R, G and B reflecting mirrors 22, 23 and 24, synthesizes them to form color images, and transmits the color images to the projecting lens 26.
The projecting lens 26 enlarges and projects the color images into the screen 27 to form projection images.
However, the three-plate type LCD has three problems as follows:
Firstly, a number of optical components increases. Especially, a relay system for compensating for optical path difference is used in addition to the illumination system. A size of the optical system also increases.
Secondly, red color purity decreases due to deficiency of R lights outputted from the lamp system. Therefore, a capability of displaying colors, namely color gamut decreases.
Third, additional expenses incur due to an increased number of LCDs which are major components of the optical system. It also causes alignment problems between the optical components.
In order to solve the foregoing problems, there has been suggested a single or two-plate type optical system as shown in FIG. 3.
FIG. 3 is a schematic view illustrating a conventional single-plate type projector.
The single-plate type projector has the similar structure as the three-plate type projector of FIGS. 1 and 2. That is, the single-plate type projector includes a lamp system, first and second optical devices, an illuminating lens, a PBS and a projecting lens like the three-plate type projector. However, differently from the three-plate type projector for spatially splitting color lights, the single-plate type projector uses a color scroll device for temporally splitting color lights such as a color drum and a color wheel. In addition, the three-plate type projector uses three LCDs, but the single-plate type projector uses only one LCD.
As depicted in FIG. 3, the single-plate type projector includes a lamp system 1 for generating lights, a first optical device 7 for homogenizing the lights from the lamp system 1, a second optical device 10 for converting a polarization property of the lights from the first optical device 7, a color wheel 28 for splitting the lights from the second optical device 10 into R, G and B lights, and scrolling them, and an optical modulation system 30 for selectively modulating the R, G and B lights according to an image signal to spatially embody color images. Here, a polarization conversion device can be used as the second optical device, which was explained above with reference to FIG. 1.
The color wheel 28 splits the polarized S-wave lights from the second optical device 10 into R, G and B lights. As shown in FIG. 4, the color wheel 28 is generally formed in a disk shape where R, G and B transmitting filters 31, 32 and 33 are sequentially aligned. While the color wheel 28 is rotated, the incident lights are selectively transmitted through the R, G and B transmitting filters 31, 32 and 33.
Accordingly, in the single-plate type projector, the lights generated in the lamp system 1 are homogenized by the first optical device 7, and polarized by the second optical device 10.
The polarized lights are split into R, G and B lights by the color wheel 28, selectively transmitted, and inputted to the optical modulation system 30.
The R, G and B lights passing through an illuminating lens 16 of the optical modulation system 30 are modulated according to the image information from an LCD 29, synthesized by a PBS 25 to form color images, and enlarged and projected into a screen through a projecting lens 26.
However, the single-plate type projector uses high-priced optical components such as the first and second optical devices for homogenizing the lights from the lamp system and polarizing the homogenized lights, which increases the production cost. In addition, it is difficult to align the plurality of optical components. A size of the optical system also increases.
Especially, the scroll device mostly used for the single or two-plate type projector, such as a color wheel or color drum transmits one of the R, G and B lights from the polarization conversion device, and reflects the other lights, thereby separately scrolling the R, G and B lights.
However, the lights reflected by the color wheel or color drum are wasted, which reduces optical efficiency. That is, the conventional scroll device does not have a means for recapturing and recycling the reflected lights.
As illustrated in FIG. 4, in the case of the color wheel having the R, G and B transmitting filters 31, 32 and 33, approximately ⅔ of incident lights are lost during the color splitting and scrolling processes.